This invention relates to nonlinear optical devices and more particularly to devices for doubling the frequency of electromagnetic radiation passing therethrough.
Optical digital data storage devices, such as compact discs, have come into widespread use. Typically, such discs are read and written to by means of a light emitted by a semiconductor laser (i.e. a laser diode). However, the light generated by semiconductor laser diodes generally falls within the lower end of the electromagnetic frequency spectrum (i.e. red or infrared). The use of higher frequency light, i.e. at the blue end of the spectrum, to read and write to optical storage medium would result in greatly increased storage density. Unfortunately, however, there are yet no practical blue semiconductor lasers. To date, the only blue lasers are large gas lasers which are obviously unsuitable for use in compact and inexpensive optical storage read/write devices.
Accordingly, a device capable of converting the light emitted by readily available semiconductor laser diodes to blue light is greatly desired. Laser diodes that emit infrared light are inexpensive and widely available. The frequency of blue light is twice that of infrared radiation. Accordingly, a device capable of doubling the frequency of infrared radiation it is greatly desired. The present invention is directed to providing an inexpensive frequency doubling device that may be used in conjunction with an infrared semiconductor laser to provide blue light suitable for use in reading and writing optical storage media.
The field of non-linear optics has provided a number of devices used as frequency doublers, generally through the means of second harmonic generation (SHG) of a fundamental frequency. U.S. Pat. No. 3,384,433 (Bloembergen) is directed to a non-linear device for converting light from first and second frequencies to a third different frequency. The device includes a series of non-linear crystals or slabs of material possessing specific lattice orientation periodically disposed along the axis of propagation of the two optical beams. Another device is described in the article entitled "Non-linear Optical Effects in Rotationally Twinned Crystals: An Evaluation of CdTe, ZnTe and ZnSe" in Revue De Physique Appliquee February 1977, Vol. 12, pages 405-409 and U.S. Pat. No. 3,988,593 (Dewey) which show devices achieving frequency doubling by passing a beam perpendicularly through a series of slabs of rotationally twinned crystals.
However, the above described approaches have suffered from a number of practical difficulties. These approaches require the frequency doubling device to have a large number (in some cases more than one hundred) of accurately grown slabs of crystals having precisely defined non-linear optical properties. The growth of such crystals is difficult and relatively expensive. Accordingly, a simpler, less expensive device for frequency doubling is desirable.